Construction, industrial, agricultural, and mining machinery such as graders, rollers, compactors, crawlers, forklifts, combines, irrigation equipment, and so forth are now often driven by a hydraulic motor and a speed reduction gear box, commonly called a final drive, on each drive wheel. One such final drive is the "Torque-Hub" disclosed in U.S. Pat. No. 3,686,978 and sold by Fairfield Manufacturing Company of Lafayette, Indiana.
Final drives for construction, industrial, agricultural and mining equipment have proven to be of significant benefit in the design, manufacture and use of this equipment. The final drive is so compact that it can often be positioned within the wheel of the vehicle or within some other driven structure with as a roller or a track. This provides exceptional design flexibility to the designer of the equipment since he does not have to provide for bulky and expensive mechanical drive structures. In addition, the wheel, track or roller can be located and supported at a position on the equipment that is best suited for its function on the equipment rather than the design necessities of powering the wheel. In addition, this final drive arrangement permits the wheels to be independently driven for very fine, precise vehicle control and also to eliminate the need for a differential. Also, since the torque delivered to the wheels is delivered separately to each wheel, no one single drive shaft need carry the total torque as in conventional drive systems, so that the drive train from the hydraulic motors to the wheel can be designed to carry only half the total torque delivered to the vehicle as a whole. Finally, the use of a final drive arrangement enables the entire drive train from the prime mover to the wheel to be completely sealed against dirt, water and any other contaminant, using few seals, and mechanically covered so that all rotating parts except the wheel are concealed for operator safety.
Our analysis of the many uses to which these useful and versatile devices have been put has revealed to us one aspect of their operation wherein improvement could be made. The speed range of hydraulically powered machinery equipped with final wheel drives is often fairly narrow and is designed primarily for the usual operating speeds, but not for high speed travel between job sites. We believe the reason for this design philosophy is primarily a matter of cost. It was known that the wide speed range that is possible with hydraulic drives required a variable pump and a variable motor, whereas the narrower speed range required only a variable pump. It was thought that the cost required to move the equipment by trailer between job sites was less than the cost of providing a wider speed range and designing the equipment for stable operation at highway speeds.
We have concluded that this design philosophy is erroneous for much of the equipment that uses final wheel drives. The improvements to the drive train and to the vehicle steering and suspension necessary to enable it to drive on the highway safely at about 45 mph are much less expensive than the cost of trailering the vehicle between job sites, and the capability of moving rapidly between different locations within a single job site saves considerable operator and machine time. These savings enable the small additional investment in the original equipment to be recovered very quickly.
The cost of designing the higher speed ranges into the hydraulic drive system is indeed quite high. However, we have concluded that a two speed mechanical speed reducer used in conjunction with a hydraulic motor and a mechanical planetary final wheel drive would provide, in the combination, a greatly improved speed range and equal or better efficiency than performing the same speed variation function only hydraulically.